At an interface between two media it is helpful to be able, at different times and/or for different applications, to determine the physical, chemical, mechanical, and molecular properties of that interface. A nonintrusive, nondestructive method to investigate the interface is desirable. Because the method should not impose restrictions based on the particular environment around that interface, an electromagnetic radiation technique utilizing visible light, ultraviolet light, infrared, x-rays, radio frequencies or any other electromagnetic radiation may be used. The frequency at which one or the other interface media is transparent will determine the type (e.g., visible, ultraviolet, etc.) of signal chosen to provide a surface specific response. For example, linear reflection optical techniques (i.e., the output frequency equals the input frequency), do not provide what is called a surface specific response.
2.1 Prior Approaches
Other techniques can be used to analyze molecular properties or behavioral properties at an interface. Unfortunately, many of them may be characterized as environmentally limited in their application. For example, such techniques may be applied only in ultra-high-vacuum, which means that nothing with a liquid or gas interface may be tested. Moreover to inspect a vacuum-solid interface, the material to be examined must be placed in a high vacuum chamber. This can often be either slow, expensive or impractical depending on the size of the particular material in question.
Other techniques which exist may require that the interface be placed in other environments such as between two solids, in which case it may be necessary to destroy the particular material to be studied. Still other techniques may require either fabrication of very exotic detection means or require signal integration times which makes them unusable in any sort of real time industrial scenario.
2.2 Historical Development
In nonlinear optics, outputs are produced at sum, difference or harmonic frequencies of the input(s). Using second-order nonlinear optical surface spectroscopy to study a surface was originally proposed in the 1960's in "Light Waves at the Boundary of Nonlinear Media" by Bloembergen and P. S. Pershan, The Physical Review, 128, p. 193 (1962). Experimental work involving second harmonic generation was also performed. However, because lasers at the time were comparatively feeble, impractical, slow, etc., there was little subsequent work done on the development of second harmonic generation or, more generally, second-order nonlinear optical (NLO) processes at surfaces until considerably later.
More recently, researchers have reviewed NLO processing and concluded that lasers had developed enough that they could be used for studying the physical and chemical properties of surfaces and interfaces. For example, a theoretical study of the physics of the interface and not its engineering aspects has been performed. See Annual Review of Materials Science, "Surface Second Harmonic Generation", Vol. 16, pp. 69-86 (1986).